1. Field of the Invention
The present invention relates to an image forming apparatus that is used as, for instance, a copying machine.
2. Description of the Related Art
In recent years, in digital copying machines having a high processing speed, the photo-conductive drum having a large diameter is used. So, it is difficult to separate a paper from the photo-conductive drum by the paper's stickiness after the transferring of the developed image onto the paper from the photo-conductive drum surface.
For instance, U.S. Pat. No. 5,585,906 (Dec. 17, 1996; Takahashi et al.) shows a method to electrostatically adsorb a paper to a transfer belt by using it as a transfer member.
In case of a belt transfer system, as a paper is conveyed as being adsorbed on a transfer belt, a paper must be separated from the transfer belt at the most downstream side of the transfer belt after conveyed. Normally, a transfer belt is supported by rollers in diameter 12-40 mm and paper can be separated by its stickiness.
However, depending on kind of paper, disturbance of image is generated (remarkable on tracing paper). This image disturbance is conspicuous when a half-tone image is printed and is generated especially on the leading edge and trailing edge of a paper in a low humidity environment (especially conspicuous at the trailing edge).
One cause of this image disturbance is considered to be unnecessary discharge generated when the trailing edge of a paper leaped when it is separated from a transfer belt. As a result of this discharge, discharge traces in a pattern like foot marks of a crow are formed on the leading and trailing edge portions of a paper as shown in FIG. 2. These discharge traces appear remarkably especially on a half-tone image portion.
It is known that this phenomenon becomes inconspicuous gradually when increasing transfer voltage (current value).
However, if the transfer voltage is increased unnecessarily, drop of transfer efficiency, uneven transfer and other defects are caused. It is therefore difficult to simply increase transfer voltage uniformly.